1. Field of the Invention
The present invention relates to a transfer apparatus which is used for image forming apparatuses or the like, specifically, to a transfer apparatus for transferring in good conditions an image formed on a recording medium onto a conveyed sheet or the like for transfer.
2. Description of the Related Art
In an image forming apparatus, for example, an electrophotographic image forming apparatus, a toner image is formed on a photosensitive member as a recording medium and transferred onto a transfer sheet such as ordinary paper. Then, in order to retain the toner image transferred onto the sheet as a permanent image, the image transferred sheet is passed through, for example, a heat fixing device to fix the transferred image, and then discharged out of the main body of the apparatus.
In such a image forming apparatus, in order to form a desired image on a sheet, it is necessary to feed the sheet to an image forming position, specifically, to a transfer position. For instance, a lot of sheets are held in a paper feeding cassette and sent one by one from the cassette to a conveying path which leads to an image forming portion, specifically, to a transfer position where the toner image formed on the photosensitive member is transferred.
For example, Japanese Unexamined Utility Model Publication JP-U-4-101567 (1992) discloses, as shown in FIG. 10, a photosensitive drum 50 of a recording medium is exposed to an optical image corresponding to record information to form an electrostatic latent image, and toner is supplied to the electrostatic latent image to obtain a visible image (a toner image). The toner image is transferred onto a conveyed transfer material 52 at a transfer apparatus 51 disposed opposite to the photosensitive drum 50.
A pre-transfer roller 53 for guiding the transfer material 52 to a transfer position of the photosensitive drum 50 is provided so as to oppose the photosensitive drum 50 before a transfer is performed. The pre-transfer roller 53 rotates to smoothly send the transfer material 52 to the photosensitive drum 50 while decreasing the running resistance of the transfer material 52. By the action of the pre-transfer roller 53, the transfer material 52 that has been sent adheres to the photosensitive drum 50. Then, the toner image formed on the photosensitive drum 50 is transferred onto the transfer material 52 as a result of corona discharge by a discharging member 55 (a corona discharging wire) of a transfer unit 54 which constitutes the transfer apparatus 51.
Accordingly, the transfer material 52 guided to the transfer position of the transfer apparatus 51 provided so as to oppose to the photosensitive drum 50 is sent by the action of the pre-transfer roller 53 in a stable condition. The pre-transfer roller 53 is disposed in order to decrease the running resistance of the transfer material 52, as described above.
The transfer material 52 is guided to the transfer position along a pre-transfer guide 56, as shown in FIG. 11, before being sent to a transfer area by the pre-transfer roller 53. The pre-transfer guide 56 is shaped into a form of comb-teeth so that a tip end thereof is positioned between the photosensitive drum 50 and the pre-transfer roller 53. The pre-transfer guide 56 is provided in order that the comb-teeth tip end prevents the rear end of the transfer material 52 from suddenly slipping off after passing by the pre-transfer roller 53 to guide the transfer material. Accordingly, the transfer material 52 is guided along the surface of the guide 56, and thereafter smoothly sent to the transfer position by the pre-transfer roller 53.
As described above, in the transfer apparatus disposed to a conventional image forming apparatus, a discharging unit (a transfer unit) which performs corona discharge from the back of the transfer material 52, the pre-transfer guide 56, the pre-transfer roller 53, a supporting portion thereof, and a post-transfer guide 57 for guiding a sheet which has been transferred are formed separately from each other. When manufacturing a transfer apparatus by assembling the separated components as shown above, it is extremely inconvenient to assemble and position them. Accordingly, the total cost including cost for manufacturing the transfer apparatus extremely increases.
Furthermore, it is required to accurately position the pre-transfer guide 56 and the discharge member (the discharge wire) 55 in relation to each other as well as the pre-transfer guide 56 and the post-transfer guide 57 in relation to each other, before a transfer is performed. If any transfer failure is caused, or transfer efficiency is lowered due to the deviation between a position where a transfer material is guided to the photosensitive drum 50 and a position where corona discharge is performed, a normal transfer may not be performed. Since the separated components as shown above are fabricated separately from each other, means for positioning or for positional adjustment is required, resulting in increase in cost.